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Background

Transformers are one of the key components in any

power systems which operates under AC principles.

The impact of it is equally important to all the

stakeholders involved (eg. DNOs, TSOs and

Manufacturers).

Transformer energisation is a regular operation intransmission and/or distribution networks and the

issues associated with it are not new. However, it is

becoming increasingly relevant due to the changes that

are shaping power networks. Frequent modifications in

the network topology due to liberalisation (e.g.

connection of distributed generation and private

networks) and the expected increase in the penetration

of offshore renewable energy are typical examples of

such drivers for this change. This technical brochure

presents a comprehensive study guide to transformer

energisation phenomenon.

General issues which are applicable to UK ESI

Transformers may be switched on and off for various

reasons. While network transformers are seldom

switched, generator transformers may be switched

more frequently depending on dispatch requirements.

Hence, transformer energisation or re-energisation is a

normal planned operation in an electric power system.

Sometimes, energising a transformer results in the

transformer drawing a relatively large initial inrush

current which decays over time to a much smaller

steady state magnetizing current. The transientmagnetising current that occurs during transformer

energisation (the inrush current) is produced by

transformer core saturation following switch-on.

Consequently, there is a voltage drop across the

network impedance and a drop in the line voltages

where the effect increases in the direction towards the

transformer.

It is shown that the inrush currents depend on the

residual flux in the transformer core and the closing

instants of the circuit breaker poles. The mechanisms

by which the RMS-voltage drop and the temporary

overvoltages are generated, in addition to the system

conditions under which these phenomena may appear

are described. The temporary overvoltages (TOV)

during transformer energisation are generated by the

interaction of the harmonic components of the inrush

currents with the resonances in the system.

 A qualitative and simplified representation of the inrush

current phenomenon is illustrated in Figure 1 for

energisation at voltage zero crossing.

Figure 1 Qualitative representation of the inrush current

and the effect of the residual flux.

Extensive information on some useful techniques that

can be implemented to mitigate the RMS-voltage drop

and/or the temporary overvoltages is also provided.

Controlling the closing times of the energising circuit

breaker (point-on-wave closing) is the most effective

technique identified, since the whole transient can be

virtually eliminated: the inrush current, the RMS-voltage

drop and the TOV. Among other techniques, reducing

the system voltage and/or adjusting the on-load tap

before energising the transformer are good ways to

reduce the inrush current and its consequences.

Transformer Energisation in Power

SystemsThis Technical Insight from CIGRE UK is based on the 2013 publication of CIGRE Technical Brochure (TB) 568.

CIGRE UK Technical Insight 568

uk cigre org

8/18/2019 Transformer Energisation

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CIGRE UK Technical Insight 568

uk cigre org

CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or

exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law.

Find out more

Founded in 1921, CIGRE, the Council on Large Electric Systems, is an international non-profit association for

promoting collaboration with experts from all around the world by sharing knowledge and joining forces to improve

the electric power systems of today and tomorrow.

For information on CIGRE UK membership, please contact Colin Ray (colin.ray@nationalgrid.com)

For further information on the topic of this Technical Insight, please contact Zia Emin (zia.emin@pbworld.com)

This TB provides exhaustive guidelines to simulate

both RMS voltage drop and TOV and to assess their

probability of exceeding pre-defined levels.

Recommendations on modelling the variouscomponents while studying transformer energisation

are also given. The assessment of the potential

damaging effects of the temporary overvoltages are

also addressed in the TB. It is shown that, for phase-to-

ground TOV lasting a few seconds or less, the surge

arresters connected between phase and ground to

protect the transformers from switching and lightning

surges are generally the most vulnerable equipment.

For longer duration TOVs, the most vulnerable

equipment to phase-to-ground TOV generally is power

transformers.

Temporary overvoltage can have deleterious effects on

electronic equipment. It is possible for the effects to be

either immediate, as in the case of stress beyond a

component's ability to withstand the voltage, or long-

term, such as slow degradation brought on by long-

term heating. There are different methods to mitigate

inrush current and TOV and some of them are more

effective than others depend on the application.

 A large section of the TB is devoted to the treatment of

uncertainty in energisation studies. Regarding the

inrush currents, they are highly dependent on tworandom parameters: the residual flux in the core before

the energisation and the circuit-breaker point-on-wave

closing times. Due to their dependency on the

uncertain parameters, the currents and voltages are

not deterministic but rather stochastic variables. Often,

however, the user is not interested in the whole

distribution of the output but in the risk of exceeding a

particular threshold limit, for instance a given RMS

voltage drop limit or the equipment overvoltage

withstand limit.

Specific issues which are applicable to UK ESI

The phenomenon of “pseudo inrush” can occur during

the recovery process following a voltage sag event,

such as after the clearance of a fault. This is aphenomenon by which transformers already in

operation are driven into saturation after a fault has

been cleared and the normal system voltage is

restored at the transformer terminals. The process is

also interchangeably referred to as “re-energization”.

This problem, which is generally more severe in weak

systems, may lead to tripping by under voltage and

over-current relays.

Furthermore, the phenomenon of sympathetic

interaction between transformers is also explained.

Whenever a transformer is energised, the asymmetry

of the generated inrush currents can drive the nearby

already energised transformers into saturation.

Specific issues which are applicable to the wider ESI

The occurrence of TOV is of particular concern during

network restoration following a system wide blackout (a

highly unlikely event in UK) because, during such

times, the network tends to have much lower resonant

frequencies and higher impedances as well as less

damping due to light loading conditions.

Practical techniques have been described in order for

the practising engineers to be able to compute this risk,

which in general is the final result of the engineering

study.

Other Relevant CIGRE Publications

CIGRE has produced a number of Technical Brochures

which will be relevant to those interested in the

energisation of transformers, including: 

CIGRE Technical Brochure 568

CIGRE Brochure 264